Mathieu Bouchard

Forest dynamics after successive spruce budworm outbreaks in mixedwood forests

In order to assess the long-term spatiotemporal influence of the spruce budworm in sub-boreal mixedwood forests, we studied the effect of three successive outbreaks in a region of western Quebec, Canada. We used dendrochronology to detect past outbreaks in three areas (111-185 ha), based on the recruitment age of balsam fir (Abies balsamea) and on growth patterns of white spruce (Picea glauca), the two main host species of this defoliating insect. We also used a series of aerial photographs taken between 1935 and 2003 to evaluate overstory mortality and post-outbreak succession patterns in these same areas. Individual outbreaks had a spatially homogenous impact on host species throughout the region, but successive outbreaks differed in intensity: the two outbreaks around 1910 and 1980 caused widespread mortality in the overstory, but an outbreak around 1945 had little impact, probably because the forest mosaic had not yet recuperated from the 1910 outbreak. No clear outbreak was detected in the later part of the 19th century. In portions of the study areas where the 1910 outbreak had a major impact, between 36% and 50% of the stands were reoccupied by balsam fir stands in the period up to the 1980 outbreak (cyclic succession), the rest being at least partly replaced by nonhost species such as Betula spp. Changes in forest composition after the 1910 outbreak were mostly associated with upper-slope positions in all study areas. The 1980 outbreak also had a higher impact than earlier outbreaks in lower-slope positions dominated by black spruce (Picea mariana)-balsam fir mixtures. These results suggest that, at the regional scale, the abundance of mature or over-mature balsam fir stands does not determine the outbreak cycle. When an outbreak occurs, however, its impact will be strongly constrained by forest characteristics such as stand composition and structure, which are themselves influenced by previous disturbances and slope position.

Enhanced Branch and Price and Cut for Vehicle Routing with Split Deliveries and Time Windows

In this paper, we study the split delivery vehicle routing problem with time windows (SDVRPTW) that is a variant of the well-known vehicle routing problem with time windows (VRPTW), where each customer can be served by more than one vehicle. We propose enhancement procedures for the exact branch-and-price-and-cut algorithm that was recently developed for the SDVRPTW. In particular, we introduce a tabu search algorithm to solve the column-generation subproblem, extensions of several classes of valid inequalities, and a new separation algorithm for the k-path inequalities. Computational results show the effectiveness of the proposed enhancements.

A proof for the positive definiteness of the Jaccard index matrix

In this paper we provide a proof for the positive definiteness of the Jaccard index matrix used as a weighting matrix in the Euclidean distance between belief functions defined in Jousselme et al. [13]. The idea of this proof relies on the decomposition of the matrix into an infinite sum of positive semidefinite matrices. The proof is valid for any size of the frame of discernment but we provide an illustration for a frame of three elements. The Jaccard index matrix being positive definite guaranties that the associated Euclidean distance is a full metric and thus that a null distance between two belief functions implies that these belief functions are strictly identical.

Tree recruitment pulses and long-term species coexistence in mixed forests of western Québec

Abstract In sub-boreal forests of northeastern North America, late-successional stands often contain a fine-scale mixture of several coniferous and hardwood tree species, including an important proportion of relatively shade-intolerant species such as white birch (Betula papyrifera) and yellow birch (B. alleghaniensis). In order to better understand the recruitment of these species, we determined the age at 1 m height of each tree in sample plots located in three stand types representative of different site conditions in a small forest mosaic undisturbed by humans. Three recruitment pulses occurred synchronously in all stand types during the periods 1835–1860, 1910–1930, and 1970–1990, the last two being related to known spruce budworm (Choristoneura fumiferana) outbreaks. These pulsed recruitment episodes were identified for birch species, but also for more shade-tolerant species such as balsam fir (Abies balsamifera) and to a lesser degree white cedar (Thuja occidentalis). White cedar was the only species for which there was some recruitment during the 1940–1960 period, but this was restricted to only one stand type located on mesic sites. These results indicate that when examined over relatively long periods of time, resource availability after canopy disturbances is an important factor for the recruitment above 1 m height for most tree species. In sub-boreal forests, periodic and synchronous mortality of balsam fir can favour the emergence of multi-species, multi-cohort stands, especially where soil conditions are adequate for the recruitment of other tree species.

Assigning multiple activities to work shifts

In some companies such as large retail stores, the employees perform different activities (e.g., cashier or clerk in a specific department) to respond to a customer demand for each activity that varies over the planning horizon and must be fulfilled as soon as possible. For a given time period, this demand translates into an ideal number of employees required for the corresponding activity. During a work shift, an employee can be assigned to several activities that are interruptible at any time and subject to operational constraints (required skills, minimum and maximum assignment durations). Given work shifts already assigned to the employees, the multi-activity assignment problem (MAAP) consists of assigning activities to the shifts such that the activity demands are satisfied as best as possible over the planning horizon. In this paper, we propose three integer programming models for the MAAP and develop various heuristics based on mathematical programming techniques. Computational results obtained on randomly generated MAAP instances show that a heuristic column generation method embedded into a rolling horizon procedure provides the best results in general.

Optimization/simulation-based framework for the evaluation of supply chain management policies in the forest product industry

This work describes a framework for the elaboration and evaluation of management policies for production and transportation supply chains in the forest product industry. The approach deals with the issue of coordination between the tactical and operational decision levels. First, we introduce LogiLab, a software system allowing to model the network and to optimize product flows in the supply chain. We than show how one can use this tactical aggregated plan to identify management policies that will guide day to day operations at the operational level. Finally, a discrete event simulation model allows assessing with more details what would be the impact of implementing these policies at the operational/execution level.

Rapid Nucleation of Iron Oxide Nanoclusters in Aqueous Solution by Plasma Electrochemistry

Progresses in cold atmospheric plasma technologies have made possible the synthesis of nanoparticles in aqueous solutions using plasma electrochemistry principles. In this contribution, a reactor based on microhollow cathodes and operating at atmospheric pressure was developed to synthesize iron-based nanoclusters (nanoparticles). Argon plasma discharges are generated at the tip of the microhollow cathodes, which are placed near the surface of an aqueous solution containing iron salts (FeCl2 and FeCl3) and surfactants (biocompatible dextran). Upon reaction at the plasma-liquid interface, reduction processes occur and lead to the nucleation of ultrasmall iron-based nanoclusters (IONCs). The purified IONCs were investigated by XPS and FTIR, which confirmed that the nucleated clusters contain a highly hydrated form of iron oxide, close to the stoichiometric constituents of α-FeOOH (goethite) or Fe5O3(OH)9 (ferrihydrite). Relaxivity values of r1 = 0.40 mM(-1) s(-1) and r2/r1 = 1.35 were measured (at 1.41 T); these are intermediate values between the relaxometric properties of superparamagnetic iron oxide nanoparticles used in medicine (USPIO) and those of ferritin, an endogenous contrast agent. Plasma-synthesized IONCs were injected into the mouse model and provided positive vascular signal enhancement in T1-w. MRI for a period of 10-20 min. Indications of rapid and strong elimination through the urinary and gastrointestinal tracts were also found. This study is the first to report on the development of a compact reactor suitable for the synthesis of MRI iron-based contrast media solutions, on site and upon demand.

A framework to optimize the restoration and retention of large mature forest tracts in managed boreal landscapes

The decreasing abundance of mature forests and their fragmentation have been identified as major threats for the preservation of biodiversity in managed landscapes. In this study, we developed a multi-level framework to coordinate forest harvestings so as to optimize the retention or restoration of large mature forest tracts in managed forests. We used mixed-integer programming for this optimization, and integrated realistic management assumptions regarding stand yield and operational harvest constraints. The model was parameterized for eastern Canadian boreal forests, where clear-cutting is the main silvicultural system, and is used to examine two hypotheses. First, we tested if mature forest tract targets had more negative impacts on wood supplies when implemented in landscapes that are very different from targeted conditions. Second, we tested the hypothesis that using more partial cuts can be useful to attenuate the negative impacts of mature forest targets on wood supplies. The results indicate that without the integration of an explicit mature forest tract target, the optimization leads to relatively high fragmentation levels. Forcing the retention or restoration of large mature forest tracts on 40% of the landscapes had negative impacts on wood supplies in all types of landscapes, but these impacts were less important in landscapes that were initially fragmented. This counter-intuitive result is explained by the presence in the models of an operational constraint that forbids diffuse patterns of harvestings, which are more costly. Once this constraint is applied, the residual impact of the mature forest tract target is low. The results also indicate that partial cuts are of very limited use to attenuate the impacts of mature forest tract targets on wood supplies in highly fragmented landscapes. Partial cuts are somewhat more useful in landscapes that are less fragmented, but they have to be well coordinated with clearcut schedules in order to contribute efficiently to conservation objectives. This modeling framework could easily be adapted and parameterized to test hypotheses or to optimize restoration schedules in landscapes where issues such as forest fragmentation and the abundance of mature or old-growth forests are a concern.

A periodic re-planning approach for demand-driven wood remanufacturing industry: a real-scale application

This article develops an experimental platform to select production planning policy in demand-driven wood remanufacturing industry. This industry is characterised by divergent co-production, alternative processes, a make-to-order philosophy and short order cycle times. Under such complex characteristics, the selection of an efficient production plan is a complex task. Previous work has failed to address all the industrial characteristics encountered in wood remanufacturing mills. After defining key performance indicators (KPIs) to measure the production plan efficiency, our methodology uses a periodic re-planning strategy based on a rolling horizon. Then, mixed-integer programming models are formulated leading to different planning approaches. Finally, the resulting decision framework is experimented to prescribe the best planning policy based on the selected KPI. Each production planning is characterised by its planning approach and factors related to the re-planning interval and the planning horizon length. Simulations are conducted using multiple best subset selections combined with an experimental design approach. Using industrial data from a wood remanufacturing mill in Eastern Canada, results indicate that the manufacturing mill should use a planning approach that minimises cost, while utilising the full system capacity. Results also quantify the benefit of using lower re-planning intervals and higher planning horizons.

Forest Structure and Understory Plant Communities Inside and Outside Tree Retention Groups in Boreal Forests

Abstract: Alternatives are being sought to the widespread use of clear-cut logging in boreal forests. Group retention harvesting is a silvicultural treatment in which well-distributed but relatively small residual forest patches (ca 10 m wide) are left inside cutover sites. The objective of this study was to compare vascular plant communities, tree species regeneration, and dead wood retention in tree retention groups and adjacent clear-cuts with soil protection. Our results indicate that plant diversity is relatively similar inside tree retention groups and the adjacent clear-cut area. This result may be explained by the important spatial variability observed among the stands, which were located in different geographical locations, the fact that soils were little disturbed during harvesting in clear-cuts and few opportunities were present for the establishment of pioneer species, and the relatively short time span since harvesting. Using a functional trait approach, we found that shade tolerance still plays a significant but relatively minor role in explaining species abundance between the 2 environments. Tree retention groups also retain a greater quantity and greater variability of dead wood materials compared with clear-cuts, and a higher abundance of regenerating trees, which is likely explained in part by the direct damage caused by the harvesting operations outside tree retention groups. Overall, retention groups do not appear to confer superior protection for late-successional plants compared with traditional clear-cut logging with soil protection. Their ecological value mostly consists in ensuring a minimal input of dead woody materials for saproxylic species.